1. Field of the Invention
This invention relates to a method of conducting the drilling of a borehole such as is used for hydrocarbon exploration and production. The well trajectory is derived from a look-ahead and look-around borehole seismic survey. Such a survey can provide very valuable information in the case of small drilling targets. This method is applicable also to other applications requiring look-ahead and look-around borehole seismic information.
2. Description of the Related Art
Vertical seismic profiling or "VSP" is a well known technique for obtaining geophysical information around a borehole (Hardage B. A., 1983, Vertical Seismic Profiling, Handbook of Geophysical Exploration, Section I, Geophysical Press, London, Amsterdam.). One or more seismic sources are positioned near the borehole at the surface and a sonde having one or more seismic detectors is lowered into the borehole on a wireline cable. The sonde is then positioned at a number of depths in the well while the sources are activated and seismic readings are taken. Suitable tools for performing such services are Schlumberger's Array Seismic Imager (ASI) or Combinable Seismic Imager (CSI) tools.
Typically VSP surveys are conducted when drilling has reached target depth or at a convenient pause in drilling operations such as an intermediate casing point, and serve to provide a diagnosis of the formations through which the borehole has been drilled.
Pinnacle reefs are high potential gas-bearing reservoirs that can be lucrative drilling targets for the oil and gas industry. They usually have a limited circular size with a diameter that can be as small as 600' to 800'. Their size is very small compared to their distance from the surface that can be greater than 16,000'. Their vertical section can be as thick as 600' which makes them attractive and preferred drilling targets in a Reef Trend. An object of this invention is to provide a technique which assists in steering drilling into a target such as a reef structure.
3D seismic represents a valuable tool to locate prospective reef development. Reef tops generally mark the interface from shale to limestone that creates a high-reflectivity event in the seismic wavefield. Surface and borehole seismic methods can detect and image those reefs that show different patterns of structure and amplitude in seismic data. These two methods have fundamental differences:
First, surface seismic images generally have less resolution than the ones obtained by borehole seismic. VSP sensors are deployed in a borehole that is characterised by a low ambient noise level for better signal recording. Also, seismic waves of a VSP survey generally have to propagate shorter distances and are less attenuated than surface seismic waves.
Second, the precision of accurately mapping the location of a reef structure in space depends on the quality of seismic stacking and migration velocity analysis. For surface seismic data, velocity analysis is based on data acquired at the surface that usually have greater errors with increasing depth. Velocity errors result in a deteriorated surface seismic image. Thus the actual position of a small pinnacle reef can be skewed both vertically and laterally in such an image. Borehole seismic, in turn, calibrates seismic velocity since is measures velocities in a well.
Third, borehole seismic has contributed successfully to the evaluation of thickness and spatial extent of pinnacle reefs by running offset Vertical Seismic Profiles (VSPs). In conventional surveys, geophones are deployed down to the drilling target depth. However, the methods used to date are mainly of diagnostic character. These aim to calibrate images obtained from surface seismic data, to provide depth information of the top and the base of a reef, and to map structural detail within a reef.
The present invention aims to overcome some or all of these problems and limitations in the existing techniques.